A mason bee is a bee of the family Osmia. They are solitary bees, they do not make honey, but they are excellent pollinators. They collect pollen to feed their babies, and are as effective as honey bees (family Apis).
Osma lignaria (Blue orchard/mason bee) is native to the United States. Honey bees are imports from Europe.
Because they don't live in a hive or produce honey, they are an excellent choice for gardens with pets, children, and people who are allergic to bees. The lack of a hive makes these bees non aggressive, and they have also earned the nickname "stingless bees".
Our lot doesn't have a spot on it that would be beneficial to honeybees. The backyard gets about 3 hours of morning sun.
We also have a hound dog. The first time I was stung by a bee was because my beagle nosed up a beehive while we were on a walk. And I was stung by LOTS of bees.
I have gotten over my fear of bees (Thanks, Nessa!) and would someday like to have a hive of honeybees, but this house & yard (and neighbors who ARE allergic) are not suitable.
For further reading:
Crown Bees (our bee supplier)